Paul B wrote:

> Sina
>
> But RTR1 is the ASBR, it is RTR2 & RTR4 which are the
> ABRs to Area 1 and are attached to the area where the
> ASBR resides(the backbone).
>
> Am I missing something ?
>

ok when an ASBR generate type 5 LSA, it's flooded in its area, now the
ABR of this area will have a intra area path to this ASBR and will
generate type 4 for all its attached area and the backbone then another
ABR in the bacbone receiving this type 4 it will have a Inter area route
to the ASBR and will generate a summary type 4 for its attached area so
you will never have an external route to the ASBR from an ABR

I was assuming that the RTR1 was learning external route from an
internal ASBR in another area and generating type 4 for area 0 and area
2
if RTR1 is the ASBR in this case RTR4 will genrate a type 4 for its
attached area But note that it has an Intra area route to the ASBR which
is in area 0

Sina

>
> Regards
>
> Paul
>
> --- Sina Mirtorabi <sinaCISCO.COM> wrote:
> > Paul
> >
> > it's not RTR4 who generates the type 4 LSA but RTR1
> > since it is the ABR
> > type 4 is generated by the ABR attached to the area
> > of an ASBR so its
> > always an intra area path from the originating ABR
> > to the ASBR
> >
> > Sina
> >
> > Paul B wrote:
> >
> > > Thanks Sina
> > >
> > > I think I explained quite badly one key element
> > which
> > > you pointed out re ASExternal advertisements. I
> > guess
> > > the key for RTR5 is not the ASExternal for the
> > route
> > > it is the cost that RTR4 advertises to reach RTR1
> > in
> > > the Type4 it generates. If it advertises the cost
> > > based on the non-backbone cost then this is why I
> > see
> > > the non-backbone to non-backbone routing occuring.
> > >
> > > Do I have this right ?
> > >
> > > Regards
> > >
> > > Paul
> > > --- Sina Mirtorabi <sinaCISCO.COM> wrote:
> > > > Paul,
> > > >
> > > > Paul B wrote:
> > > >
> > > > > Can anyone help clarify what should happen in
> > this
> > > > > scenario please with and without 1583
> > > > compatibility.
> > > > >
> > > > > Which path is correct to this external
> > destination
> > > > > which is advertised as a type 1 external from
> > ASBR
> > > > > RTR1. Specifically I am considering the
> > > > non-backbone
> > > > > intra-area vs backbone intra-area rule.
> > > > >
> > > > > My question relates to which next hop RTR4
> > should
> > > > > install and advertise a metric for.
> > > > >
> > > > > This is the scenario.
> > > > >
> > > > > 48.0.1.0/24 (External Route)
> > > > > |
> > > > > |
> > > > > RTR1------Area 0 ---RTR2---Area1
> > > > > | \ ---- |
> > > > > | \ |
> > > > > Area 2 Area0 RTR5
> > > > > | \- |
> > > > > | \ |
> > > > > RTR 3--Area2----Rtr4---Area1-|
> > > > >
> > > > > >From the perspective of RTR5
> > > > > the cost to 48.0.1.0 is 9 via RTR4-RTR1
> > > > > 10 via RTR2-RTR1
> > > > > 13 via RTR4-RTR3-RTR1
> > > > >
> > > > > Is it correct for RTR4 to advertise a metric
> > based
> > > > on
> > > > > a path via RTR3 because this subscribes to the
> > > > > non-backbone intra-area rule from its
> > perspective
> > > > or
> > > > > should it advertise the backbone path/metric
> > via
> > > > RTR1
> > > > > ?
> > > >
> > > > RTR4 is not advertising any thing, type 5 is
> > > > generated by ASBR ( I guess
> > > > RTR1 in your figure ) and are flooded untouched
> > AS
> > > > wide ( except Stub
> > > > and NSSA )
> > > > so RT4 will just flood it to the area 1 and RT5
> > will
> > > > pick up one of the
> > > > path through RTR4 or RTR2 based on the cost
> > > > however once a packet reach RTR4 it will chose
> > one
> > > > of the path based on
> > > > the 1583 compatibility, if its disabled it will
> > > > choose the intra-area
> > > > non backbone path which is through area 2
> > > >
> > > > >
> > > > >
> > > > > I am not clear as to whether it is possible to
> > > > route
> > > > > from non-backbone to non backbone.
> > > > >
> > > > > Because RTR4 is an ABR in area 1 & Area 2 is
> > it
> > > > > correct for RTR4 to subscribe to the
> > intra-area
> > > > > non-backbone rule and advertising the
> > route/cost
> > > > via
> > > > > RTR3 through area 2 ? i.e. should RTR4
> > advertise
> > > > the
> > > > > cost to RTR5 based on going through the
> > > > non-backbone
> > > > > area 2, and not its backbone area connection
> > even
> > > > > though it is receiving on a non-backbone
> > interface
> > > > ?
> > > > >
> > > >
> > > > again RTR4 is not the one who generates the LSA
> > > >
> > > > >
> > > > > I am not clear as to whether it is legal to
> > route
> > > > > through multiple non-backbone areas if you
> > receive
> > > > a
> > > > > packet from a non-backbone area.
> > > > >
> > > >
> > > > if you have 2 non backbone area attached to an
> > ABR
> > > > they could
> > > > communicate directly through this ABR depending
> > on
> > > > the path cost
> > > > however you cannot communicate between multiple
> > non
> > > > backbone area if
> > > > they are not attached to the same ABR because an
> > ABR
> > > > consider only the
> > > > summary from the backbone and not from a non
> > > > backbone area
> > > >
> > > > Sina
> > > >
> > > > >
> > > > > Thanks
> > > > >
> > > > > Paul
> > > > >
> > > > >
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